In pursuit of a best possible shift quality, most modern automatic transmissions use one-way clutches, often in series with a multi-plate clutch for establishment of intermediate transmission gear ratios. Such arrangements inherently offer a superior shift quality with the least complicated control system. However, such designs often require yet another friction device, parallel to the one-way device, for coast braking, thereby making the transmission larger, heavier and costlier. For this reason, many current transmissions employ friction brake bands as grounding devices in place of one-way clutches.
Friction brake systems using bands are of compact construction and cause less parasitic losses than multiple plate clutches when running in a released state. They have an important functional advantage over multi-plate clutches in that the torque capacity of a friction brake band in one direction is much higher than in the opposite (coast) direction; about twice as high for a single wrap band and nearly four times for a double wrap band. This makes the calibration for a power-on band-to-clutch shifts, for instance, significantly easier than for a clutch-to-clutch shift. Yet, the obtained band-to-clutch shift quality is generally not as good and as consistent as can be obtained with a one-way way roller or sprag clutch. In general, poorly calibrated shifts, besides being uncomfortable to the driver and passengers, reduce the life of friction elements and can cause a costly premature failure of the transmission. The object of this invention is to provide a friction brake band system that makes shifts that are as smooth and precisely timed as the shifts obtained with one-way roller or sprag-type clutches.
The brake band system of this invention is based on the principle of making the device self-regulate its torque capacity in proportion to the torque that it carries. The friction band and control system of this invention based on such self-regulating principle will automatically release at the moment the transmitted torque reduces to zero and is about to change its direction, as happens during a gear shift from a lower ratio to a higher one. The underlying basic self-regulating principle used here is analogous to the principle used in my previous three self-synchronizing clutch inventions, which are covered in the following U.S. patents:
1. Multi-mode Clutch for Change-speed transmissions
U.S. Pat. No. 5,031,746 issued on Jul. 16, 1991.
2. Bi-directional Multi-mode Clutch for Change-speed Unit for Automatic Change Speed transmissions
U.S. Pat. No. 5,106,348 issued on Apr. 21, 1992.
3. Gear-thrust Controlled Multi-mode Clutch for Power Transmissions
U.S. Pat. No. 5,281,190 issued on Jan. 25, 1994.